1. Go to Steam library, right click on DCS: A-10C and view product key.
2. Copy product key to clipboard.
3. Create and save it into a notepad. (optional but I did)
4. Uninstall the Steam version and manually delete any old left over files.
5. Go to the following directory and delete the DCS Warthog folder.
C:\Users\YOURNAME\Saved Games
6. Go to this link and download the bottom official torrent file for DCS World.
http://www.digitalcombatsimulator.com/index.php?ProductId=21&end_pos=137&scr=product&lang=en
7. Go to this link and download the bottom official torrent file for DCS: A-10C Warthog.
http://www.digitalcombatsimulator.com/index.php?scr=product&ProductId=8&end_pos=136&lang=en
8. Once installed DCS World install DCS: A-10, the installer will find DCS World automatically.
9. Start DCS World. When in-game you will be prompted to enter the product key when you select a mission or go to the multilayer screen. Just simply enter the product key you copied from Steam in step 2.
That's it!